The performance characteristics of polymeric materials are determined not only by their composition but also by their molecular architecture. For copolymers, various properties such as melt viscosity, glass transition temperature, and modulus are often a function of the distribution of the different monomeric units along the polymeric chain. Conventional radical polymerization methods have limited utility in synthesizing polymers with precise architectural and structural characteristics.
Controlled radical polymerization methods have been developed that allow the preparation of polymers with well-defined molecular weight, polydispersity, topology, composition, and microstructure. These methods are based on the use of special polymerization mediators, which temporarily and reversibly transform propagating radicals into dormant and/or stable species. These reversible transformations are typically either accomplished by reversible deactivation or by reversible chain transfer. Some of the methods that involve controlled radical polymerization through reversible transformations include iniferter methods, nitroxide mediated polymerization (NMP) methods, atom transfer polymerization (ATRP) methods, and reversible addition-fragmentation (RAFT) methods.
The terms “iniferter” and “photoiniferters” refer to molecules that can act as an initiator, chain transfer agent, and terminator. Various iniferters were discussed in Otsu et al., Makromol. Chem., Rapid Commun., 3, 127-132 (1982). The compound p-xylene bis(N,N-diethyldithiocarbamate) (XDC) has been used to form various acrylic-based block copolymers such as those described in European Patent Applications 0286376 A2 (Otsu et al.) and 0349270 A2 (Mahfuza et al.).
Some polymeric materials have been formed by applying a layer of a crosslinkable composition to the surface of a substrate. The crosslinkable composition can contain a pre-polymer plus additional monomers and a crosslinking agent. Crosslinked compositions can be prepared by exposing the crosslinkable composition to actinic radiation such as ultraviolet radiation. Such polymeric materials and processes are described in U.S. Pat. No. 4,181,752 (Martens et al.), U.S. Pat. No. 4,330,590 (Vesley), U.S. Pat. No. 4,329,384 (Vesley et al.), U.S. Pat. No. 4,379,201 (Heilmann et al.), U.S. Pat. No. 5,506,279 (Babu et al.), U.S. Pat. No. 5,773,836 (Bennett et al.), and U.S. Pat. No. 5,773,485 (Bennett et al.).